Erythrocytes, leukocytes and platelets are the essential cells of the human hematopoietic system. The primary function of erythrocytes, also known as red blood cells, is to transport hemoglobin, which in turn carries oxygen from the lungs to tissues. Oxygenated hemoglobin gives the erythrocytes a red color. Leukocytes, also referred to as myeloid cells, are a heterogenous group of cells that mediate immune responses and which include granulocytes, monocytes, and lymphocytes. These cells are found predominately in the blood, bone marrow, lymphoid organs and epithelium. Leukocytes are referred to as white blood cells because of a lack of natural pigment which gives the cells a whitish or transparent appearance.
Platelets are small (2-4 .mu.m) anucleate disk-like cells which play an important role in blood coagulation. Platelets are derived by cytoplasmic fragmentation of the precursor stem cells, megakaryocytes, found in bone marrow. After formation, platelets leave the bone marrow and travel through the spleen and into the blood, with approximately one third of the platelets becoming sequestered in the spleen. The platelets which are transported to the blood, circulate for approximately seven to ten days.
Platelets which are normally present in human blood at a concentration of 150,000-400,000 per microliter play a crucial role in hemostasis, or the regulation of bleeding. When the level of platelets falls below normal in a subject, the risk of hemorrhage increases in the subject. The condition associated with low levels of platelets is referred to as thrombocytopenia.
Ordinarily when the level of circulating platelets decreases a feedback mechanism is initiated which results in increased production in the number, size, and ploidy of megakaryocytes. This mechanism, in turn, causes the production and release into the circulation of additional platelets. Although the feed back regulation of platelet levels is ordinarily sufficient to maintain a normal level of platelets in the circulation, several physiological conditions are capable of causing a significant imbalance in the level of platelets. Such conditions result in either thrombocytopenia or thrombocytosis (a condition caused by an increased level of platelets in the blood).
At least three physiological conditions are known to result in thrombocytopenia: a decreased production of platelets in the bone marrow; an increased splenic sequestration of platelets; or an accelerated destruction of platelets. In conventional therapies in order to successfully treat thrombocytopenia, one must first identify which mechanism is causing the decrease in platelet levels and then treat the subject by administering a drug or instituting a procedure which will eliminate the underlying cause of the platelet loss.
A loss of platelets due to decreased production of bone marrow, may be established by the examination of a bone marrow aspirate or biopsy which demonstrates a reduced number of megakaryocytes. A decreased production of bone marrow may result from myelosuppression as a consequence of gamma irradiation, therapeutic exposure to radiation, or cytotoxic drug treatment. Chemicals containing benzene or anthracene and even some commonly used drugs such as chloramphenicol, thiouracil, and barbiturate hypnotics can cause myelosuppression, resulting in thrombocytopenia. Additionally, rare bone marrow disorders such as congenital amegakaryocytic hypoplasia and thrombocytopenia with absent radii (TAR syndrome) can selectively decrease megakaryocyte production, resulting in thrombocytopenia.
Splenic sequestration of platelets can cause an increase in spleen size. Splenic sequestration can often be determined by bedside palpation to estimate splenic size. An increase in spleen size, or splenomegaly, is typically caused by portal hypertension secondary to liver disease, splenic infiltration with tumor cells in myeloproliferative or lymphoproliferative disorders, or macrophage storage disorders such as Gauchers disease. Splenectomy is often used to increase platelet counts in cases of excessive splenic sequestration.
Thrombocytopenia resulting from accelerated destruction of platelets is generally the cause of decreased levels of platelets in the blood when impaired production of bone marrow and splenic sequestration have been ruled out. The accelerated destruction of platelets is caused by either an immunologic disorder or a non-immunologic disorder. Immunologic thrombocytopenia can be caused, for example, by autoimmune disorders such as idiopathic thrombocytopenic purpura (ITP), viral or bacterial infections, and drugs. Non-immunologic thrombocytopenia is caused by vasculitis, hemolytic uremic syndrome, thrombotic thrombocytopenic purpura (ITP), disseminated intravascular coagulation (DIC) and prosthetic cardiac valves. Chronic ITP is often treated with high doses of steroids, intravenous gamma globulins, splenectomy, and even immunosuppressive drugs. Each of these therapeutic modalities provides only temporary relief and is associated with serious side effects. Additionally, approximately 20 percent of the chronic ITP patients do not respond to any of the known treatments.
Factors which induce proliferation and differentiation of megakaryocytes have been proposed as therapeutic treatments for increasing platelet counts, regardless of the mechanism causing thrombocytopenia. Thus far three such factors have been identified, but have not yet been well characterized. A recent report has suggested that the three factors, C-MpL ligand, thrombopoietin, and megakaryocytes colony stimulating factor (MK-CSF) are the same molecule (Wendling F. et al., C-MpL ligand is a humoral regulator of megakaryocytopoesis, Nature, 369; p. 571-574 (1994)). Interleukin 11 has been shown to induce megakaryocytopoesis in an animal model. It is unclear whether these compounds will be effective clinical treatments for thrombocytopenia.
There presently exists a need for therapeutic methods for increasing platelet levels or preventing significant decreases in platelet levels. Ideally, the therapy should eliminate thrombocytopenia in individuals exhibiting thrombocytopenia or prevent thrombocytopenia in individuals at risk of developing thrombocytopenia. Such therapy preferably should be easy and safe to administer and require few diagnostic tests to follow the course of treatment. The patient preferably should be allowed to be treated on an ambulatory basis, thereby reducing the hospital visits while still allowing an improved quality of life. Such therapy preferably should be effective for all types of thrombocytopenias regardless of the underlying physiological condition causing platelet depletion.